Nitride semiconductors are used in a blue LED, an LED and LD used for multi-color, and the like which are used as light sources of illumination and back light.
In nitride semiconductor manufacturing, bulk single crystal production is difficult, and thus GaN is grown on a different kind of substrate such as a sapphire substrate, or a SiC substrate by use of MOCVD (metal organic chemical vapor deposition). Since exhibiting excellent stability in high-temperature ammonium atmospheres in epitaxial growth steps, a sapphire substrate is particularly used as a substrate for growth. However, the sapphire substrate is an insulating substrate and does not show conductivity, whereby electrodes cannot be disposed with the substrate intervening therebetween.
Therefore, a nitride semiconductor on a sapphire substrate generally employ a structure in which two electrodes of p and n types are disposed on the same face side of the substrate by etching the nitride semiconductor after epitaxial growth until an n-type gallium nitride is exposed, and by forming an n-type contact on the etched face.
In the structure in which two electrodes of p and n types are disposed on the same face side as described above, however, currents tend to be concentrated on a mesa part close to the n electrode, whereby the ESD (electrostatic breakdown) voltage cannot be raised. In addition, it is difficult to uniformly inject currents into an active layer and also difficult to cause the active layer to emit light uniformly. Furthermore, the p and n electrodes need electrodes for wire bonding, respectively, on the same face side, so that the effective light emitting area is reduced as compared with a nitride semiconductor on a conductive substrate which requires only either one of the electrodes for wire bonding to be disposed. This increases the chip (element) area, decreasing the number of chips to be produced from one wafer. Moreover, because sapphire has high hardness and a hexagonal crystal structure, the sapphire substrate needs to be separated into chips by scribing when sapphire is used for a substrate for growth, making manufacturing steps complicated and the yield bad.
Meanwhile, a flip chip type has been proposed as a structure of a nitride semiconductor element using a sapphire substrate for improving light output efficiency. In this type of structure, a p-type layer is disposed under the sapphire substrate, and thus light is emitted from the sapphire substrate. The flip chip type has an advantage of being brighter owing to the effects that the escape cone is larger since the difference of the refractive index between sapphire and air is smaller than that between GaN and air, and that the amount of light emitted from lateral directions also is larger because the sapphire substrate, although being made thinner by grinding or the like, still has a residual sapphire thickness from 60 μm to 80 μm. However, even in this flip chip type, both the electrodes of p and n types are present on the same face side, whereby any of the above problems cannot be solved.
On the other hand, if electrodes are disposed to be opposite to each other using a conductive substrate or the like, the above problems can be solved. However, SiC of a conductive substrate usable for nitride gallium is costly at present and the amount of light absorption becomes large when an impurity is doped in a substrate to make the substrate conductive.
In order to solve these problems, a method is used that includes peeling a sapphire substrate, exposing an n-type gallium nitride layer, and then forming an n electrode thereon. For example, there is a laser lift-off (hereinafter referred to as LLO) method that includes forming a compound crystal layer, serving as a nitride semiconductor, on a sapphire substrate with a GaN buffer layer interposed therebetween; irradiating the resultant substrate from the sapphire substrate side with an excimer laser beam of approximately 300 nm or less at a several hundred mJ/cm2; and decomposing the GaN buffer layer to peel the sapphire substrate. This method can produce a chip equivalent to a chip produced by use of a GaN substrate, so that electrodes can be disposed to be opposite to each other, thereby being capable of technically solving the above problems.
However, implementation of LLO decomposes a GaN buffer layer to generate N2 gas and thus applies shear stress to the GaN buffer layer and the nitride semiconductor, whereby the border part of a region irradiated with laser light frequently has cracks. For the prevention of cracks caused by generation of this N2 gas, for example as illustrated in FIG. 13, isolation trenches 24 are formed by dry-etching to reach a sapphire substrate 21 and to have a size allowing a GaN buffer layer 22 and a nitride semiconductor 23 to be divided into elements. The GaN buffer layer 22 is formed on the sapphire substrate 21 and functions as a separating layer, and the nitride semiconductor 23 is grown on the GaN buffer layer 22 and has a light emitting region.
Next, when a laser beam is radiated from behind the sapphire substrate 21, the GaN buffer layer 22 absorbs a laser beam to be decomposed into Ga and N and generate a N2 gas. However, the N2 gas is discharged from the isolation trenches 24, thereby this prevents excess stress due to the N2 gas from being applied to the crystal layer of the nitride semiconductor 23.
Patent Document 1: JP-A 2003-168820